Concentrating solar collector

ABSTRACT

The invention concerns a concentrating collector comprising a cover ( 1 ) transparent to the sun&#39;s rays, a cylindro-parabolic reflector ( 2 ), a coolant element ( 3 ) and a transformer ( 5 ) for reducing losses of energy by emissivity, said transformer ( 5 ) not being contacted with said coolant element ( 3 ).

FIELD OF THE INVENTION

The present invention relates to a concentrating solar collector.

PRIOR ART

The general operating principle of a solar collector is to capture solarradiation to transform it into heat or electricity.

There are two main types of collectors, “flat-plate” collectors and“concentrating” collectors. Flat-plate collectors transform the solarradiation into heat over the entire surface of the collector withoutconcentrating it, whereas concentrating collectors concentrate the solarradiation at the focal point by way of a reflector or optical meansusing a magnifying glass effect.

Among concentrating solar collectors, the following distinction is made:on the one hand, there are collectors that use a reflector in the shapeof a revolving parabola. These collectors concentrate the solar rays atthe focal point of the parabola on a Stirling engine, an engine in whichthe heat is transformed into movement, a movement which is transformedinto electricity by means of an alternator.

On the other hand, there are collectors which use a cylindro-parabolicreflector and which concentrate the solar rays onto a converter thatenables the solar rays to be transformed into heat. The converter alsocomprises a tube in which a heat-transfer fluid circulates. This fluidwill enable the stored heat to be gathered in a storage reservoir or toretrieve it via a heat exchanger.

The converter must have not only a high level of absorption of theentire spectrum of solar radiation, but also possess a reducedemissivity, i.e. reduced reemission of heat. In addition, inconcentrating solar collectors, the surface of the converter isgenerally weaker than that of the opening of the collector.

Such concentrating solar collectors are well known. The converters useddisplay either a rectangular cross section or a circular cross section,while the reflectors, for their part, can be hemi-cylindrical orcyclo-parabolic.

Documents EP 0 033 054, U.S. Pat. No. 3,321,012 and U.S. Pat. No.4,024,852 describe solar collectors in which the converter displays arectangular cross section. As described in document EP 0 35 033 054, thereflector can have the shape of a continuous curve composed of at leastfive arcs of a circle joining one another tangentially, but it can alsohave a hemi-cylindrical shape, as described in document U.S. Pat. No.3,321,012, or else even a cyclo-parabolic shape as described in documentU.S. Pat. No. 4,024,852.

Documents U.S. Pat. No. 4,059,094, AT 402 114 and AT 344 375 describesolar collectors in which the converter possesses a circular crosssection. As described in documents AT 402 114 and AT 344 375, thereflector can have a hemi-cylindrical shape, or else even acyclo-parabolic shape as described in document U.S. Pat. No. 4,059,094.

Different solutions have been suggested to improve the efficiency ofconcentrating solar collectors. Some of these solutions are based on theuse of different materials to make the constituent elements of acollector in order to increase the yield of the transfer of solar energyinto heat.

Other solutions, as described in documents AT 344 375 and AT 402 114,suggest maximising the quantity of solar radiation received by thecollector through using an “absorber”, an auxiliary element that is anintegral part of the converter. This absorber captures part of theindirect solar radiation and transfers the supplementary heat, thusabsorbed, to the heat-transfer fluid.

Nevertheless, these solutions produce an energy yield that is still muchtoo low and have the disadvantage of not limiting heat losses caused bya phenomenon of heat reemission, particularly at the level of theconverter.

OBJECTS OF THE INVENTION

The present invention aims to supply a concentrating solar collectorthat does not present the disadvantages of the prior art.

It aims in particular to supply a concentrating solar collector withimproved efficiency. It also aims to supply a solar collector, which canbe used in the form of solar panels and which can, in addition, be usedto generate electricity, heating, air-conditioning or waterdesalination.

The expressions “collector”, “cell”, “captor” are used interchangeablyin the following account of the invention.

SUMMARY OF THE INVENTION

The present invention describes a concentrating solar collectorcomprising a cover that is transparent to solar rays, a reflector with acylindro-parabolic cross section, a heat-transfer element and atransformer reducing energy losses through emissivity, said transformernot being in contact with said heat-transfer element.

According to particular embodiments, the invention includes one orseveral of the following characteristic features:

-   -   the heat-transfer element is arranged inside a vacuum tube.    -   the transformer is fixed to the cover and/or the reflector        and/or the vacuum tube.    -   the transformer is a plate lying in the plane of symmetry of the        cross section of the reflector.    -   the transformer is a carbon or silica plate or a metal plate.    -   the transformer comprises lamellae.    -   the transformer is covered with a substance of high thermal        emission.    -   the cover of the collector is made of a transparent synthetic        material or of solar glass.    -   the volume is a volume filled with a gas or a liquid.    -   the collector comprises in addition an opening enabling the gas        or the liquid to be injected into or evacuated from the volume.    -   the heat-transfer element has a rectangular cross section, a        circular cross section, an oval cross section or a square shape.    -   the heat-transfer element is a metal tube or a tube made out of        a plastic material.    -   the surface of the heat-transfer element is partially or        entirely covered with a substance that has a high absorption of        the solar spectrum and a low thermal emission.    -   the heat-transfer element has the characteristics of a heat        pipe.    -   the heat-transfer element is located at the focal point of the        solar collector.    -   the heat-transfer element contains a heat-transfer liquid or        gas.    -   the collector comprises in addition a thermal insulator.    -   the thermal insulator is chosen from a polyurethane foam, a        polystyrene foam, rock fibre and glass fibre.    -   the collector comprises in addition at least one sun tracking        means.

The present invention in addition describes the use of the solarcollector in the form of solar panels.

The present invention also describes the use of the solar collector togenerate electricity or heating or air-conditioning or waterdesalination.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic view of a particular embodiment of the solarcollector according to the invention.

FIG. 2 shows a schematic view of a particular embodiment of thetransformer.

FIG. 3 shows a schematic view of a multitude of solar collectorsaccording to the invention forming a solar panel.

DETAILED DESCRIPTION OF DRAWINGS

In the concentrating solar collector according to the invention, thesolar radiation is concentrated on a heat-transfer element 3 by theintermediary of a longitudinal reflector with a cylindro-parabolic crosssection 2.

The solar radiation received by the heat-transfer element 3 is absorbed,however, part of this absorbed energy is reemitted into the volume 7.This reemitted energy can pass through the cover 1, in which case itwill be definitely lost, or it can be absorbed by the cover 1 of thesolar collector.

The originality of the present invention lies in the use of atransformer 5 enabling part of the solar radiation, preferably theindirect solar radiation, to be used to reduce the energy losses throughemissivity and to limit the quantity of radiant energy.

“Radiant energy” means energy emitted during the natural phenomenoncalled thermal emissivity, a phenomenon during which any material hit bythe solar radiation emits an infra-red radiation by cooling down. Thus,radiant energy comprises the energy reemitted by the elements that makeup the solar collector, in particular through the heat-transfer element.

The solar collector according to the invention comprises a cover 1, areflector with a cylindro-parabolic cross section 2, a heat-transferelement 3 as well as a transformer 5.

According to a particular embodiment of the invention, the assembly ofthe reflector 2 and the heat-transfer element 3 is supported on, orincluded in, a layer of a thermal insulator 6 resting in a cradle 8 or aframe, on which the cover 1 rests (FIG. 1). Said cradle 8 has preferablya square or rectangular shape.

The cover 1 is preferably made of a material that is transparent tosolar rays, which has a weak degree of reflection and a hightransmission. Advantageously, the material is a transparent syntheticmaterial or a solar glass.

“Solar glass” means a glass which has a low iron content and the surfaceof which is treated so as to reduce the reflection effects. The surfacetreatment can be achieved through the application of a layer, or film,of Teflon.

Advantageously, the interior surface of the cover 1 can be covered witha substance reflecting the infra-red rays.

The reflector 2 has a cylindro-parabolic cross section and alongitudinal shape. It enables the concentration of the solar radiationto be obtained on the heat-transfer element 3, which is situated at thefocal point of the parabola.

The reflector 2 has a highly reflective surface. It preferably has thecharacteristics of a mirror. With this aim in view, the reflector 2 canbe made of a rigid material, for instance glass, on which a layer of areflecting substance is applied.

Preferably the reflector 2 is made of aluminium.

The heat-transfer element 3, in which a heat-transfer fluid circulates,preferably has a rectangular cross section, a circular cross section, anoval cross section or a square shape. It can be a metal tube or a tubemade out of a plastic material resistant to the operating temperaturesof the solar collector. Said heat-transfer element is preferably made ofcopper or aluminium.

The surface of the heat-transfer element 3 is preferably covered,entirely or partially, with a layer of a material with a high absorptionof the solar spectrum and a low thermal emission. However, in the caseof a partial covering of said heat-transfer element 3, the substancewith a high absorption of the solar spectrum and a low thermal emissionwill have to cover the surface of said element 3, which is turnedtowards the opening of the parabola.

The heat-transfer element 3 contains a heat-transfer gas or liquid. Theheat-transfer gas can be air. The heat-transfer liquid can be water,with or without additive, or, for operating temperatures above 200° C.,said fluid can be a stable oil, which does not disintegrate at hightemperatures.

Advantageously, the heat-transfer element 3 has the characteristics of aheat pipe.

A heat pipe is a hollow tube containing a heat-transfer fluid undervacuum, which is chosen in function of its condensation/vaporisationtemperature. The heat pipe enables the heat to be standardised anddisplaced as rapidly as possible, thanks to a phase-changing phenomenon.The solar energy concentrated by the reflector within the converterenables the heat-transfer fluid to be vaporised. This energy is thenretrieved at the level of a colder zone, in a heat exchanger, forinstance, through the condensation of the heat-transfer fluid.

Preferably, the heat-transfer element 3 is arranged inside a vacuum tube4. Said tube 4 enables energy losses of the heat-transfer element 3 tobe limited.

Advantageously, said vacuum tube 4 is made of a transparent material,preferably a solar glass or a borosilicate glass. Said tube 4 can becovered with a layer of a substance that has anti-reflection properties.

The thermal insulation of the solar collector according to the inventionis ensured, on the one hand, by the presence of the cover 1 and, on theother hand, by a thermal insulator 6.

The insulator 6 is preferably a polyurethane or polystyrene foam, rockfibre or glass fibre.

The transformer 5 enables the losses of heat due to the emissivity ofthe heat-transfer element 3 to be diminished and thus plays a role inthe insulation of the solar collector by creating a thermal barrieragainst the infra-red reemissions of the heat-transfer element 3. Bycapturing part of the solar radiation, the transformer 5 heats up andreemits radiant energy which is transferred to the volume 7.

The transformer 5 is not in contact with the heat-transfer element 3,neither directly nor indirectly. There is no solid-solid contact betweensaid transformer 5 and said heat-transfer element 3. In the embodimentin which the heat-transfer element 3 is arranged inside a vacuum tube 4,the transformer 5 can be in contact with said tube 4 provided that it isnot in contact with the heat-transfer element 3.

The transformer 5 is fixed to the cover 1, and/or the reflector 2,and/or the vacuum tube 4. Said transformer 5 is situated in the plane ofsymmetry of the cross section of the reflector, along the entire lengthof the solar collector. More precisely, if the heat-transfer element 3,which is located at the focal point of the cylindro-parabolic reflector2, is situated at the bottom of the parabola, as represented in FIG. 1,said transformer 5 will be arranged under the plate of the cover 1,whereas if the heat-transfer element 3 is situated under the cover 1,said transformer 5 will be arranged between the heat-transfer element 3and the bottom of the parabola. For all the intermediate situations,i.e. all the positions the heat-transfer element 3 can adopt along theplane of symmetry of the cross section of the reflector 2, thetransformer 5 can have the shape of two distinct elements, which arearranged on either side of the heat-transfer element 3, one towards thecover, the other towards the bottom of the parabola.

The transformer 5 is a good heat conductor and preferably a carbon orsilica plate, or else a metal or metal alloy plate or strip. Said metalcan for instance be copper, aluminium, silver, gold, tungsten, brass,tin, or an alloy of one or several of the metals cited above.

In a preferred embodiment of the invention, the transformer 5 is made ofany material which is covered by a substance with a high thermalemission. Said substance can also have a high absorption of the solarspectrum and a high thermal emission. This can be any darkish substance,preferably of a metallic nature. It can be carbon in any of its forms,soot or graphite for instance, or silica. It can be a metal such ascopper, aluminium, anodised aluminium, silver, gold, tungsten, brass,tin, or an alloy of one of several of the metals cited above.

The transformer 5 preferably has a rectangular shape.

Advantageously, the transformer 5 can further comprise lamellae that canbe interlinked (FIG. 2). This particular embodiment of the element 5enables a better diffusion and distribution of the radiation emitted bythe transformer 5 in the volume 7. This enables the turbulences whichmay be present in the volume 7 to be reduced.

The volume 7 corresponds to the space existing between the reflector andthe cover of the collector. Said volume 7 contains a gas or a fluid,capable of absorbing the infra-red radiation.

Preferably, the gas filling said volume 7 is carbon dioxide or airenriched with carbon dioxide or enriched with steam. This could also beone or several CFCs, methane or sulphur hexafluoride.

The pressure of the gas of the volume 7 can be inferior, equal orsuperior to the atmospheric pressure. To this effect, the solarcollector can in addition comprise an opening enabling the fluid to beinjected into or evacuated from the volume 7.

The cradle 8, on which the assembly of the cover 1, the reflector 2 andthe heat-transfer element 3 is supported, is preferably made of a metal,aluminium for instance, or of wood, or a composite material, or elseeven a combination of these materials.

The solar collector according to the invention advantageously comprisesat least one sun tracking means. Said means enables the course of thesun to be tracked and for the position of the collector to be adjustedin all directions in order to obtain an optimal solar exposure, i.e. inorder to enable the collector to adopt an ideal position with regard tothe sun with a view to capturing the maximum of energy. This is achievedby mechanical or hydraulic means.

The solar collector according to the invention is preferably used in theform of solar panels (FIG. 3).

The solar collector according to the invention is preferably used forthe generation of electricity, heating, air-conditioning or for waterdesalination.

1. A concentrating solar collector comprising a cover (1) that istransparent to solar rays, a reflector (2) of cylindro-parabolic crosssection, said cover (1) and said reflector (2) defining a volume (7),and a heat-transfer element (3), characterised in that it furtherincludes a transformer (5) for reducing the emissivity energy losses,said transformer (5) being covered with a substance having a highthermal emission, in that said transformer (5) absorbs some of the solarradiation and reemits heat into said volume (7) and in that saidtransformer (5) is not in contact with said heat-transfer element (3).2. The concentrating solar collector according to claim 1, wherein theheat-transfer element (3) is arranged inside a transparent vacuum tube(4).
 3. The concentrating solar collector according to claim 1, whereinthe transformer (5) is fixed to the cover (1) and/or to the reflector(2) and/or to the vacuum tube (4).
 4. The concentrating solar collectoraccording to claim 1, wherein the transformer (5) is a plate lying inthe plane of symmetry of the cross section of the reflector (2).
 5. Theconcentrating solar collector according to claim 1, wherein thetransformer (5) is a carbon or silica plate or a metal plate.
 6. Theconcentrating solar collector according to claim 1, wherein thetransformer (5) comprises lamellae (9).
 7. The concentrating solarcollector according to claim 1, wherein the cover (1) of said collectoris made of a transparent synthetic material or of solar glass. 8.Concentrating solar collector according to claim 1, wherein the volume(7) is filled with a gas or with a liquid.
 9. The concentrating solarcollector according to claim 8, wherein further comprises an openingenabling the gas or the liquid to be injected or evacuated from thevolume (7).
 10. The concentrating solar collector according to claim 1,wherein the heat-transfer element (3) has a rectangular cross section, acircular cross section, an oval cross section or a square shape.
 11. Theconcentrating solar collector according to claim 1, wherein theheat-transfer element (3) is a metal tube or a tube made out of aplastic material.
 12. The concentrating solar collector according toclaim 1, wherein the surface of the heat-transfer element (3) ispartially or entirely covered with a substance having a high absorptionof the solar spectrum and a low thermal emission.
 13. The concentratingsolar collector according to claim 1, wherein the heat-transfer element(3) displays the characteristics of a heat pipe.
 14. The concentratingsolar collector according to claim 1, wherein the heat-transfer element(3) is located at the focal point of the solar collector.
 15. Theconcentrating solar collector according to claim 1, wherein theheat-transfer element (3) contains a heat-transfer liquid or gas. 16.The concentrating solar collector according to claim 1, wherein saidcollector further comprises a thermal insulator (6).
 17. Theconcentrating solar collector according to claim 16, wherein theinsulator (6) is chosen from a polyurethane foam, a polystyrene foam,rock fibre and glass fibre.
 18. The concentrating solar collectoraccording to claim 1, wherein further comprises at least one suntracking means.
 19. Use of the solar collector according to claim 1, inthe form of solar panels.
 20. Use of the solar collector according toclaim 1 for the generation of electricity or for heating or for airconditioning or for water desalination.